Optical rotary encoders are widely used to detect the position and/or speed of a rotating object. FIG. 1 schematically shows a basic construction of an optical detection section of the optical rotary encoder. As shown in FIG. 1, the optical detection section has a code plate 1 constituting a rotational slit plate, a rotating shaft 2, a fixed slit 3, a light receiving section 4, and a light emitting section 5 as basic elements. The light emitting section 5 incorporates a lens or the like for making a beam parallel as well as one or a plurality of light emitting devices (for example, LEDs).
Part of a beam (stationary beam) L emitted from the light emitting section 5 is detected by the light receiving section 4 after passing through light transmittingportions of the code plate 1 and the fixed slit 3 in succession, and almost all of the remaining beam is obstructed by a light intercepting portions of the code plate 1 or the fixed slit 3.
FIG. 2 shows a general cross-sectional construction of the code plate 1, fixed slit 3, and light receiving section 4 used in the rotary encoder having the basic construction shown in FIG. 1. As shown in FIG. 2, a movable slit in the code plate 1 has a function of periodically distributing the stationary beam L emitted from the light emitting section 5 and converting it into a plurality of discrete beams moving according to the motion of an object being checked (for example, a rotor shaft of motor).
For the code plate 1, light intercepting portions 12 and light transmitting portions 13 are formed periodically on the surface (one surface or both surfaces) of a transparent board 11 with a pitch of a half of a predetermined reference pitch IP. As the transparent board 11, an optical material such as a glass plate is used. The light intercepting portions 12 and the light transmitting portions 13 are formed by chromium depositing the whole surface of the transparent board 11 and then by selectively removing the chromium deposition film by etching. The portions from which the chromium deposition film is removed by etching serve as light transmitting portions 13, and the remaining portions serve as light intercepting portions 12.
The construction and manufacturing method of the fixed slit 3 are the same as those of the code plate 1. Specifically, light intercepting portions 32 and light transmitting portions 33 are formed periodically on the surface (one surface or both surfaces) of a transparent board 31 with a pitch of a half of the reference pitch IP, which is the same as that of the code plate 1. As the transparent board 31, an optical material such as a glass plate is used, and on the surface thereof the light intercepting portions 32 are formed by using a chromium deposition film or the like. As in the case of the movable slit, the chromium deposition film can selectively be removed by etching to form board regions corresponding to the light transmitting portions 33. Alternatively, the light transmitting portions 33 may be formed by machining, for example, punching the board 31 made of a light intercepting material.
The light receiving section 4 has light receiving devices (for example, photodiodes) 42 arranged on a board 41 with a pitch of a half of the reference pitch IP. When the rotating shaft 2 connected to a rotor of motor or the like is rotated, the code plate 1 is rotated, so that the rotational position of the light transmitting portion 13 formed on the code plate 1 changes. Accordingly, the discrete beam is scanned, and the lapping relationship with the light transmitting portion 33 formed on the fixed slit 3 changes periodically.
The light incident on the light receiving device (light sensing zone) 42 is converted into an electrical signal, while the light incident on the region (light non-sensing zone) where the light receiving device (light sensing zone) 42 is not provided is not converted into an electrical signal. As a result, of the quantity of light contained in the to-be-scanned beam, the proportion of the quantity of light incident on the light receiving device (light sensing zone) 42 on the light receiving section 4 changes periodically. The periodical electrical signal thus formed is processed by a publicly known processing circuit. It is to be noted that FIG. 2 shows a state in which there is established a positional relationship such that the light transmitting portions 13 of the code plate 1 agree with the light transmitting portions 33 of the fixed slit 3.
The optical rotary encoder having the optical detection section of such a conventional construction has several problems as described below.
Problem 1: The stationary beam from the light emitting section is made the movable discrete beam having periodical light and shade by the light intercepting/transmitting function of the movable slit, and then a light detection signal is obtained according to the position of movable slit (the relative position with respect to the fixed slit or light receiving device). Therefore, the utilization efficiency of light is poor. That is, of the light emitted from the light emitting section, at least a half thereof (the hatched portion in FIG. 2) cannot contribute at all to the formation of signal. Specifically, at the time when the light emitted from the light emitting section is converted into the movable discrete beam, almost a half of the quantity of light has already been wasted, so that an efficient signal output cannot be obtained.
Problem 2: In order to form a light and shade lattice having the light transmitting portions and light intercepting portions on the code plate 1 and fixed slit 3, troublesome and costly processes such as chromium deposition, etching, and machining are needed, which increases the cost of the whole optical rotary encoder.
Problem 3: In order to attach the code plate to the rotating shaft member, a plurality of attaching portions continuous with the code plate is needed. It is actually difficult to configure the attaching portions integrally (as one member) with a body portion (disk member) of the code plate, so that the whole code plate is made up of two or three members. For this reason, the code plate is easily affected by errors of fabrication accuracy and assembly accuracy of these parts, especially the alignment accuracy for alignment with the shaft member. Also, the burden of assembly work for decreasing these errors is heavy.
FIGS. 3a to 3d show code plates of the conventional optical rotary encoder. FIGS. 3a and 3b show an example of code plate made up of two members, and FIGS. 3c and 3d show an example of code plate made up of three members.
In the example of code plate made up of two members as shown in FIGS. 3a and 3b, a disk DS constituting the body portion of the code plate 1 is joined to a collar portion HL of a hub HB serving as an attaching portion to a shaft member (not shown). As the material of the disk DS, glass is used. In the region indicated by hatching in the front view, a code element (lattice pattern of transmitting portions and intercepting portions) as shown in FIG. 2 is formed by chromium deposition or the like. Also, the hub HB including the collar portion HL is made of a metal such as aluminum or brass, and an adhesive AD suitable for bonding of metal to glass is used for the joining of the collar portion HL.
The hub HB has a hollow construction, and as shown in the front view, a screw hole SH engaging with a fixing bolt (not shown), an edge portion HG, and an inner peripheral step portion HD are provided.
In the example of code plate made up of three members as shown in FIGS. 3c and 3d, a disk DS constituting the body portion of the code plate 1 is lapped on a collar portion HL of a hub HB serving as an attaching portion to a shaft member (not shown), and locked reinforcingly with a washer WS. As the material of the disk DS, metal is used, and in the region indicated by hatching in the front view, a code element (lattice pattern of transmitting portions and intercepting portions) as shown in FIG. 2 is formed by metal etching or the like. Also, the hub HB including the collar portion HL is made of a metal such as aluminum or brass.
As is the case of code plate made up of two members, the hub HB has a hollow construction, and as shown concentrically with the washer WS in the front view, a screw hole SH engaging with a fixing bolt (not shown), an edge portion HG, and an inner peripheral step portion HD are provided.
Thus, neither of the constructions shown in FIGS. 3a and 3b and FIGS. 3c and 3d are an integral construction in which the disk DS constituting the body portion of the code plate 1 is integral with the hub HB serving as an attaching portion. Therefore, when the disk DS is assembled to the hub HB (joining using adhesive, locking using washer), it is not easy to secure the alignment relationship (aligned state) between them. As a result, measures are taken such that alignment is performed with a heavy burden when the encoder is assembled, or faulty products are rejected in the inspection process at the sacrifice of a low yield.
The inventor of the present invention has made a proposal for solving problems 1 and 2 of the above problems 1 to 3 (PCT/JP98/00726). According to this proposal, a code plate using a lens element group arranged periodically is used as a means for converting the stationary beam from the light emitting section into a movable discrete beam group.
FIGS. 4a, 4b and 5 are views for illustrating one example of a code plate used in the aforementioned improvement proposal. In FIGS. 4a and 4b, a sectional view and a front view of an improved code plate denoted by reference numeral 6 are drawn, respectively, in a simplified form. The code plate 6 is provided with many lens elements 61 arranged periodically with a predetermined pitch in the circumferential direction. The lens element 61, which plays a role of movable slit (generation of movable discrete beam group) in the conventional code plate, is formed on one surface or both surfaces of the code element. By the rotation of the rotating shaft member 2 connected to a rotor shaft of motor or the like, these lens elements 61 are rotated around a center axis S.
FIG. 5 is a view for illustrating a generating operation of a movable discrete beam group in the case where the code plate 6 is used, in the same form as that of FIG. 2. The code plate 6 shown in this figure is made of a plastic such as acrylic resin and polycarbonate, and formed with many convex lens elements 61 on the light outgoing side thereof. The condensing power (refracting power) of each lens element 61 is designed so that the beam width of a movable beam MF is decreased to a half of the reference pitch IP when the movable beam MF reaches the light receiving device 42. Between the movable beams MF are formed shadow regions SD where the light does not pass through.
When the rotating shaft 2 of the code plate 6 connected to a rotor of motor or the like rotates, the code plate 6 is rotated, so that the rotational position of each lens element 61 changes synchronously by the same quantity in the same direction. Accordingly, the movable beam MF moves, and scans the surface on which the light receiving devices 42 are arranged.
As a result, of the quantity of light contained in the movable beam MF, the proportion of the quantity of light reaching the light receiving device 42 changes periodically. The quantity of light detected by each light receiving device 42 is converted into an electrical signal changing with that period, which is processed by a publicly known processing circuit. It is to be noted that FIG. 5 shows a state in which a positional relationship such that the quantity of light of the movable beams MF incident on the light receiving devices 42 is at a maximum (almost the whole quantity) holds between the code plate 6 and the light receiving device 42.
According to this improvement proposal, as is seen from the comparison between FIG. 2 and FIG. 5, in the conventional construction (FIG. 2), almost a half of the quantity of light is wasted when the stationary beam from the light emitting section is converted into the movable beams, while in the improved construction (FIG. 5), the waste of the quantity of light due to the interception of light does not occur when the stationary beam from the light emitting section is converted into the movable beams. Therefore, this construction can provide a signal output with a double efficiency in principle as compared with the conventional construction. Also, such a process as chromium deposition and metal etching is not needed. This construction is advantageous in terms of material cost and ease of fabrication.